ON COLLOID CHEMISTRY AND iTs INDUSTRIAL APPLICATIONS. 73 
viscosities of the two varieties in the torsion viscosimeter, a 20 per cent. 
solution of the hard gum indicating a retardation of 60 to 70 deg., whereas 
the soft gum showed a retardation of only 28 to 33 deg. The difference in 
these two varieties has not yet been determined, the amount of mineral 
matter and the acidity being practically the same. It was found, how- 
ever, that gum from tapped trees had a higher viscosity than that which 
exuded naturally (‘4th Report Wellcome Research Laboratories, Khar- 
toum’). ~ A solution of gum arabic yields an opaque white gel with basic 
lead acetate—no doubt, due to the adsorption of PbH,0,. It becomes 
more glutinous on addition of borax and is not precipitated by tannic acid. 
am insoluble in liquid phenol and in pyridine, in which gelatine is readily 
soluble. 
On adding alcohol to a solution of the gum, it is precipitated, but more 
readily in presence of hydrochloric acid. The precipitate has been regarded 
as ‘arabic acid,’ but this cannot be the case, since the mineral matter 
adheres very strongly to the carbohydrates, and, unless the solution is 
dilute, the gum is precipitated almost unaltered. 
Thus a solution precipitated once by alcohol in presence of dilute 
hydrochloric acid yielded a product containing 2-73 per cent. of mineral 
matter, while after three precipitations it still contained 2-24 per cent. 
In presence of sulphuric acid the precipitate contained 2-30 per cent. of 
mineral matter, while, in presence of ammonium oxalate, the precipitate 
contained 1-29 per cent. The proportion of galactan and araban did not 
appear to differ markedly from the original gum. After precipitating in 
this way, however, the gum does not dissolve to a clear sol, but is more or 
less turbid, and after about three precipitations with alcohol it ceases to be 
precipitated, but remains as a turbid sol, from which it deposits on addition 
of acids or electrolytes. On the other hand, Rideal and Youle (‘ Jour. Soc. 
Chem. Ind.’ 1891, 60) tound a difference in the optical rotation between the 
gum precipitated (amounting to about half the original) and that which 
remained in solution, from which they concluded that at least two sub- 
stances were present. O'Sullivan obtained a similar result with ‘ pure 
arabin.’ 
_ Attempts have been made to utilise this reaction for estimating the gum 
In syrups (as also the amount of precipitate obtained with lead acetate in 
alcoholic solution), gum arabic yielding 85 per cent. of ‘ pure gum’ caleu- 
lated free from ash (A. C. Chauvin, ‘ Ann. Falsif.’ 1912, 5, 27-30, also 
De. Roques and G. Sellier, ‘ Ann. Chim. Analyt.’ 1911, 16, 218-220). 
An interesting case of the mutual precipitation of two colloids is afforded: 
by the interaction of gum arabic and gelatin, under certain conditions, 
which have been closely studied by F. W. Tiebackx (‘ Z. Chem. Ind. 
Kolloide,’ 1911, 8, 198-201, and 1911, 9, 61-65). The precipitation takes: 
place in acid solution only within certain limits depending upon the: 
strength of the solutions. Thus, with 2 c.c. of a 0-5 per cent. gelatin 
solution and 2 ¢.c. ot a 2 per cent. solution of gum, coagulation takes place: 
with 2 c.c. of N/40 HCl, whereas with 2 c.c. of N/25 or N/125 acid, the: 
solution, after becoming turbid, clears again. With a solution containing. 
Q-5 per cent. of gelatin and 0-5 per cent. of gum arabic, a turbidity is. 
produced when the acidity = -002 N.HCI; on further addition of acid a. 
precipitate occurs which passes again to a turbidity and at a concentration: 
of 0-01 N the liquid becomes clear again. Increasing the percentage of 
gelatin and gum, or raising the temperature, restricts the action, until at a. 
